As semiconductor devices for controlling large current, though power semiconductor devices using Si (silicon) as a semiconductor material have conventionally been used, the improvement of their performance is difficult since Si has a limit in electrical and physical characteristics. Accordingly, development of power semiconductor devices with use of wide gap semiconductor materials having good electrical and physical characteristics compared to Si are currently proceeding. The wide gap semiconductor materials are typified by SiC (silicon carbide) having an energy gap of 2.2 eV to 3.2 eV. An insulated-gate bipolar transistor (IGBT) that is a voltage-controlled semiconductor device with use of the SiC is disclosed in, for example, Material Science Forum Vols. 338-342 (2000), pp. 1427-1430. This SIC-IGBT is shown in FIG. 7.
In FIG. 7, a p-type SiC buffer layer 102, a p-type SiC base layer 103, an n-type SiC base layer 104, and a p+ type SiC emitter layer 105 are sequentially formed by epitaxial growth method on an n+ type SiC substrate 101 having an emitter electrode 113 formed on the lower surface and connected to an emitter terminal 113a. A trench 109 reaching the base layer 103 is formed in the central portion of the SiC-IGBT, and a gate electrode 111 connected to a gate terminal 111a is provided in the trench 109 via a gate insulating film 106. On both the sides of the SiC-IGBT, a collector electrode 115 which is in contact with the base layer 104 and the emitter layer 105 are provided, and the collector electrode 115 is connected to a collector terminal 115a. 
Upon application of a voltage to between the gate electrode 111 and the collector electrode 115 so as to make the potential of the gate electrode 111 negative, electric fields are given to the gate insulating film 106 placed in between a portion of the base layer 104 forming a lateral wall of the trench 109 and the gate electrode 111. As a result, in the vicinity of a contact surface of the n-type base layer 104 in contact with the gate insulating film 106, an n-type conductivity is inverted to a P type. Since a channel for current flow is formed in a portion of the base layer 104 which is an inversion layer inverted to the P type, the channel is referred to as “inversion-type” channel. Through the channel, current flows between the collector electrode 115 and the emitter electrode 113.
In the case of the SiC-IGBT, there is a problem that the inversion layer has low channel mobility. This is considered to be because a surface state is present in the interface between SiO2 used as the gate insulating film and SiC so that the holes flowing through the inversion layer at on time are captured by the surface state. Further, it is also considered that roughness of the interface causes the holes as carriers to stop contributing to the conductivity, which causes the low mobility of the holes in the channel. Because of these reasons, channel resistance and on-voltage tend to increase.    (Patent Document 1) JP H10-256529 A    (Patent Document 2) JP H10-27899 A    (Non-patent Document 1) Trans Tech Publication (Switzerland) Material Science Forum Vols. 338-342 (2000), PP1427 to 1430